Because silicon-carbide (SiC) semiconductor has a higher dielectric breakdown field and a larger saturated drift velocity of electron and thermal conductivity than those of a silicon semiconductor, research and development are made for realizing a power device capable of operating in higher speed at a higher temperature than those of the conventional silicon device. Above all, an attention has been attracted to the development of a high-efficient switching element used in a power source for driving a motor of a power-assisted bicycle, an electric vehicle, a hybrid car and the like. To realize such a power device, a single-crystal silicon-carbide substrate having smooth surface for forming a high quality silicon-carbide semiconductor layer by epitaxial growth is necessary.
Furthermore, a blue laser diode has attracted an attention as a light source for recording information in high density, and additionally, needs to a white diode as a light source in place of a fluorescent lamp or an electric bulb are increasing. Such a light-emitting element is prepared using a gallium nitride (GaN) semiconductor, and a single-crystal silicon-carbide substrate is used as a substrate for forming a high quality gallium nitride semiconductor layer.
High processing accuracy is required in flatness of a substrate, smoothness of a substrate surface and the like to the single-crystal silicon-carbide substrate used in such use applications. Furthermore, high cleanability is required regarding a residue such as an abrasive or the like derived from a polishing agent. However, because a silicon-carbide single crystal has extremely-high hardness and excellent corrosion resistance, workability in preparing a substrate is poor, and it is difficult to obtain a single-crystal silicon-carbide substrate having high smoothness while maintaining high polishing rate. Furthermore, even in the removal of the abrasive, because the silicon-carbide single crystal has excellent corrosion resistance, a method of removing an abrasive residue by lift-off using a chemical such as hydrofluoric acid as used in the cleaning of a silicon substrate is difficult to be applied. Therefore, it is difficult to obtain a substrate surface having high cleanliness.
In general, a smooth surface of a single crystal semiconductor substrate is formed by polishing. In the case of polishing silicon-carbide single crystal, the surface thereof is mechanically polished using an abrasive such as diamond or the like that is harder than silicon carbide as an abrasive material to form a smooth surface. In such a case, fine scratches according to a particle size of the diamond abrasive are incorporated in the surface of the single-crystal silicon-carbide substrate polished with the diamond abrasive. Furthermore, because an affected layer having mechanical strain is generated on the surface, the smoothness of the surface of the substrate is not sufficient as is.
In the production of a single crystal semiconductor substrate, chemical mechanical polishing (hereinafter referred to as “CMP”) technique has been used as a method for smoothening the surface of a semiconductor substrate after mechanically polishing. CMP is a method of converting the surface of a material to be processed to an oxide or the like by utilizing a chemical reaction such as oxidation and removing the oxide formed using an abrasive having hardness lower than that of the material to be processed, thereby polishing the surface. This method has the advantage that an atomic step-and-terrace structure comprising atomic steps and terraces derived from a crystal structure is formed without generating strain on the surface of a material to be process, and extremely-smooth surface in atomic level can be formed.
The formation of a silicon-carbide semiconductor layer on a single-crystal silicon-carbide substrate by epitaxial growth is performed by depositing silicon atoms and carbon atoms by a thermal CVD method on an extremely-smooth surface in atomic level on which an atomic step-and-terrace structure has been formed by the CMP. In such a case, the front edge of the atomic step becomes the origin of epitaxial growth. Therefore, to obtain a high quality silicon-carbide semiconductor layer free of crystal defect, as surface properties required in a single-crystal silicon-carbide substrate, not only an atomic step-and-terrace structure derived from a crystal structure is formed, but high processing accuracy is required in the shape of the atomic step formed. Particularly, it is necessary in the front edge portion of the atomic step that crystal defect derived from mechanical damage by polishing is suppressed.
In the present description, the “atomic step-and-terrace structure” means a micro step-like structure comprising a plurality of flat “terraces” provided so as to be parallel to each other through step difference along a principal surface of a single crystal substrate and “atomic steps” that are step difference parts connecting the terraces. A linear site at which the upper edge of the atomic step contacts the terrace is defined as a “front edge portion of an atomic step”. The “terrace”, “atomic step” and “front edge portion of an atomic step” are further described hereinafter.
To form a high quality silicon-carbide semiconductor layer, there is a proposed method of conducting CMP by a colloidal silica slurry or a colloidal silica slurry containing an oxidizing agent after diamond polishing to form a high smoothness surface having an atomic step-and-terrace structure derived from the crystal structure, and further conducting etching by a gas-phase method (e.g., see Patent Document 1). In Patent Document 1, in the case where a silicon-carbide semiconductor layer is film-formed without conducting the etching treatment, step bunching occurs by an oxide formed extremely-near the surface of a substrate after CMP, however, by conducting the etching treatment, only a surface oxide layer generated by CMP can be removed while maintaining high smoothness of the surface of a substrate after CMP and crystal defect such as step bunching can be suppressed.
However, in Patent Document 1, although the formation of an atomic step-and-terrace structure derived from the crystal structure is considered, the influence of the edge shape of an atomic step and the crystal defect, to epitaxial growth of a crystal is not considered at all. Furthermore, merely suppressing the crystal defect of a silicon-carbide semiconductor layer by etching is not sufficient to obtain a high quality semiconductor layer. Further, higher polishing rate is required to be realized from the standpoint of cost.
A polishing composition containing a silica adhesive, an oxidizing agent (oxygen donor) such as hydrogen peroxide and vanadate is conventionally proposed as a polishing agent for polishing the surface of a single-crystal silicon-carbide substrate in high polishing rate and smoothly (e.g., see Patent Document 2).
However, in the polishing composition of Patent Document 2, there was the problem that a polishing rate to a single-crystal silicon-carbide substrate is low and time required for polishing is very long. There was further problem that although an atomic step-and-terrace structure is formed on the surface after polishing, the front edge portion of an atomic step becomes a shape having crack and dent due to mechanical damage at polishing and crystal defect occurs. Furthermore, there was a problem that silica abrasives that could not be removed by cleaning remain on a substrate and the abrasive residue causes crystal defect of a semiconductor layer epitaxially grown on the surface of a substrate after polishing.